World plastics production increases by 5–6% annually and is projected to reach 250 million tons by 2020. And polyethylene terephthalate (PET) market is the fastest growing one among them . There are many methods of waste PET depolymerization. Physical-mechanical and thermal recycling is the most used way of polyether utilization, but produced product can't be used in food industry . On the other hand solvolysis provides to create monomers from waste PET. According to depolymerization agent's type there are hydrolysis, alcoholysis, glycolysis, aminolysis, etc [3-6].
In our previous work  we proved that crude glycerol is an effective PET depolymerization agent. And this method is insensitive to impurities and different plastics in the feedstock. The aim of this work is to study kinetic regularities of the process.
Crude glycerol, waste PET and ethanol were used as a feedstock. Crude glycerol contained 53.5 wt.% of glycerol, 9.0 wt.% of fatty acids methyl ethers (FAMEs) and 37.5 wt.% of soaps (as potassium stearate). Before the depolymerization PET bottles were washed in water, then cut into pieces and dried. The process was carried out in a 250 ml three-neck glass reactor equipped with a mechanical stirrer and a thermometer. The prepared amount of PET flakes was loaded into the reactor at temperature of the reaction. The reaction was carried out at atmospheric pressure and a maximum reaction time of 200 minutes. After that the reactor was cooled in air and excess of ethanol to the reaction mass was added and the mixture was stirred. Then the resulting suspension was filtered and the white crystalline compound was washed with fresh ethanol and dried in air. The filtrate contained ethanol, glycerol, FAMEs and other liquid products. Ethanol was removed from the filtrate by heating in a rotary evaporator at 65-70°C and 50-70 mm Hg within 3 hours.
Results and Discussion
During this work the general scheme of the process was proposed (figure 1). There are two main processes: formation of potassium intercalates and PET saponification by potassium soaps. First one take place due to KOH presence in crude glycerol. This hydroxide was formed by partial soaps hydrolysis during storing. The second process proceeds on the PET surface and limited by it. Further both potassium intermediates turns to oligomer products with different molecular weights and then react with potassium soaps with formation of dipotassium terephthalate. A mathematical model of the process was proposed (figure 2) and all rate constants were determined. It was discovered that all constants decreasing with increasing PET/soaps molar ratio. At PET/soaps molar ratio 1.0 and more there is no formation of dipotassium terephthalate. These data are confirmed by molecular weight of oligomers changing. It has increased more than twice with PET/soaps molar ration increasing from 0.5 to 1.5.
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Figure 1 - General scheme of the process
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Figure 2 - Mathematical description of the process, where [KOH], [PET], [KSt], [TFKa], [Int 1], [Oligo1], [Oligo2], [Oligo3] - concentrations of potassium hydroxide, PET, potassium soaps, dipotassium terephthalate, potassium intercalate and oligomers with different molecular weight respectively
The general scheme and mathematical model of the process was proposed. It was discovered that PET/soaps molar ratio strongly influences on the rate of reactions as well as molecular mass of the oligomers. At high ratio dipotassium terephthalate couldn't be formed due to low rate constant. So at these conditions different monopotassium terephthalic oligomers are the main products.
This work was financially supported by the Russian Foundation for Basic Research (Scientific Project No. 18-29-24009).
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|Publication||Impact Factor journals|
|Affiliation of speaker||D.I. Mendeleev University of Chemical Technology of Russia|
|Position of speaker||PhD student|